Structural and functional evolution of metallacrowns.

New and efficient approaches to the semisynthesis of taxol and its C-13 side chain analogs by means of β-lactam synthon method

A comprehensive overview of recent literature from 2003 concerning advances in enantioselective copper catalysed 1,4-addition of organometallic reagents to alpha,beta-unsaturated compounds is given in this critical review. About 200 ligands and catalysts are presented, with a focus on stereoselectivities, catalyst loading, ligand structure and substrate scope. A major part is devoted to trapping and tandem reactions and a variety of recent synthetic applications are used to illustrate the practicality and current state of the art of 1,4-addition of organometallic reagents. Finally several mechanistic studies are discussed (162 references).

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Recent advances in enantioselective copper-catalyzed 1,4-addition.

Aluminum and gallium amidinate complexes, {RC(NR‘)2}MMe2 (R, R‘ = alkyl; M = Al, Ga), react with the “cationic activators” [Ph3C][B(C6F5)4] and B(C6F5)3 to yield cationic Al and Ga alkyl species whose structures are strongly influenced by the steric properties of the amidinate ligand. The reaction of acetamidinate Al complexes {MeC(NR‘)2}AlMe2 (R‘ = iPr, 1a; R‘ = Cy, 3a) with 0.5 equiv of [Ph3C][B(C6F5)4] or B(C6F5)3 yields {MeC(NR‘)2}2Al2Me3+ (R‘ = iPr, 2a+; R‘ = Cy, 4a+) as the B(C6F5)4- or MeB(C6F5)3- salts. X-ray crystallographic analyses establish that 2a+ and 4a+ are double-amidinate-bridged dinuclear cations, in which the two metal centers are linked by μ-η1,η1 and μ-η1,η2 amidinate bridges. NMR studies show that 2a+ undergoes two dynamic processes in solution:  (i) a μ-η1,η1/μ-η1,η2 amidinate exchange and (ii) Me exchange between the two metal centers. The reaction of {MeC(NiPr)2}GaMe2 (1b) with 0.5 equiv of B(C6F5)3 yields {MeC(NiPr)2}2Ga2Me3+ (2b+), whose structure and dynamic properties are sim...

Synthesis and Structures of Cationic Aluminum and Gallium Amidinate Complexes

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Infragravity waves: From driving mechanisms to impacts

This paper describes the synthesis, characterization, and synthetic application of new a-amino dialkylaluminum ester enolates. With simple imines, these in situ prepared aluminum enolates afford trans-3-amino-2-azetidinones 5 and 6 with good de’s (72-92%) in excellent yields when an (small) excess of dialkylaluminum chloride is present. Transmetalation of LiO(RO)C=C(H)NR1R2 (2, R = R1 = R2 = alkyl) with (R3)2AICI (R3 = alkyl) affords pure (R3),Al0(RO)C=C(H)NR1R2 (3) in quantitative yields. Upon addition of a second equivalent of (R3),AICI, stable adducts (R3),AI0[(RO)C=C(H)NRlR2]+AI(R3)2C1 (3’) are formed. In both types of enolates, the ester enolate anions are N,O-chelate bound to the metal centers through AI-0 and dative AI-N bonds. Consequently, the configuration of these enolates is Z. A crystal structure determination of 3b (R = R’ = R2 = R3 = Me) shows a dimeric associate, having a planar skeleton, consisting of two symmetry-related AIOCCN units ( A 1 4 = 1.853 (3) A, AI-N = 2.260 (3) A), that are connected via AlQAl bridges ( A l e h = 2.055 (3) A). The aluminum centers are five-coordinated with a slightly distorted trigonal bipyramidal geometry. Enolate 3b crystallizes as colorless crystals in the space group P21/n with a = 8.095 ( 1 ) A, b = 13.747 ( 1 ) A, c = 9.641 (2) A, B = 104.41 (I)’, V = 1039.1 (3) A3, 2 = 2, and T = 295 K. Full-matrix least-squares refinement on Fconverged at R = 0.056 and R, = 0.057. A crystal structure determination of 3b’ (R = R1 = R2 = R3 = Me) shows a dinuclear monomeric adduct, consisting of a planar AIOCCN unit ( A 1 4 = 1.870 (2) A, AI-N = 2.039 (2) A) to which a Me2AlCl molecule is coordinated through a (dative) A I 4 bond of 1.895 (2) A. The aluminum centers are four-coordinated with a distorted tetrahedral geometry. The chlorine atom is located in a semibridging position, but the long AI-CI distance of 3.260 (1) A is indicative for a nonbonding interaction. Enolate 3b’ crystallizes as pale yellow crystals in the space group P2,/n with a = 9.456 ( 1 ) A, b = 14.145 (1) A, c = 12.208 (1) A, @ = 103.78 ( 1 ) O , V = 1585.9 (2) A’, 2 = 4, and T = 295 K. Full-matrix least-squares refinement on F converged at R = 0.05 1 and R, = 0.043. The structures of enolates 3 and 3’ in solution as well as the reactions of these enolates with imines 4 is discussed. It is shown that both dimeric enolates 3 as well as monomeric adducts 3’ may act as the reactive intermediates in the one-pot synthesis of 3-amino-2-azetidinones 5 and 6, which is characterized by the reactions of in situ prepared a-amino aluminum ester enolates with imines. Furthermore, on the basis of the structural features of 3b and 3b’ the trans stereoselectivity, observed in these reactions, is explained in terms of two highly ordered transition states, constructed from 2-aluminum enolates and E-imines. ,

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Synthesis and molecular structures of novel α-amino organoaluminum ester enolates. Key intermediates in the selective formation of trans-3-amino-2-azetidinones

Two field data sets of near-bed velocity, pressure, and sediment concentration are analyzed to study the influence of infragravity waves on sand suspension and cross-shore transport. On the moderately sloping Sand Motor beach (≈1:35), the local ratio of infragravity wave height to sea-swell wave height is relatively small (HIG/HSW 0.4), most sand is suspended during negative infragravity velocities, and qIG is offshore directed. The infragravity contribution to the total sand flux is considerably larger and reaches up to ≈60% during energetic conditions. On the whole, HIG/HSW is a good indicator for the infragravity-related sand suspension mechanism and the resulting infragravity sand transport direction and relative importance.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/2016JF003878

Cross‐shore sand transport by infragravity waves as a function of beach steepness

The synthesis and properties of novel (organo)zinc enoaltes of N,N-disubstituted glycine esters : Molecular structure of [EtZnOC(OMe)=C(H)N(t-Bu)Me]4

The stereoselective synthesis of cis- and trans-b-lactams via α-amino ester enolates and 1-aza-4-hetero-1,3-butadiene systems. Molecular structure of EtOC(O)C(H)Net2)C(H)(NH)(-t-Bu)C(H)=N(t-Bu)ZnCl2: an unexpectedly isolated intermediate

Cristallisation dans P1 avec affinement respectivement jusqu'a 0,0480 et 0,0735. Les longueurs et angles de liaison, ainsi que les conformations sont tres voisins dans (I) et (II). Le cycle phenyl sur N(1) est coplanaire avec le couple Al a cinq membres dans les deux composes tandis que le cycle phenyl sur N(z) fait un angle de 78,1 o dans (I) et de 69,9 o dans (II)

Structures of [N-(p-methoxyphenyl)-2-(p-methoxyphenylimino)propylamino-N,N']dimethylaluminium (I) and dimethyl[N-(p-tolyl)-2-(p-tolylimino)propylamino-N,N']aluminium (II)

F. H. Van Der Steen

Papers

Synthesis and molecular structures of novel α-amino organoaluminum ester enolates. Key intermediates in the selective formation of trans-3-amino-2-azetidinones

Cross‐shore sand transport by infragravity waves as a function of beach steepness

The synthesis and properties of novel (organo)zinc enoaltes of N,N-disubstituted glycine esters : Molecular structure of [EtZnOC(OMe)=C(H)N(t-Bu)Me]4

The stereoselective synthesis of cis- and trans-b-lactams via α-amino ester enolates and 1-aza-4-hetero-1,3-butadiene systems. Molecular structure of EtOC(O)C(H)Net2)C(H)(NH)(-t-Bu)C(H)=N(t-Bu)ZnCl2: an unexpectedly isolated intermediate

Structures of [N-(p-methoxyphenyl)-2-(p-methoxyphenylimino)propylamino-N,N']dimethylaluminium (I) and dimethyl[N-(p-tolyl)-2-(p-tolylimino)propylamino-N,N']aluminium (II)